Defect reduction in seeded aluminum nitride crystal growth

ABSTRACT

Bulk single crystal of aluminum nitride (AlN) having an areal planar defect density≦100 cm −2 . Methods for growing single crystal aluminum nitride include melting an aluminum foil to uniformly wet a foundation with a layer of aluminum, the foundation forming a portion of an AlN seed holder, for an AlN seed to be used for the AlN growth. The holder may consist essentially of a substantially impervious backing plate.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/015,957, filed on Jan. 17, 2008 now U.S. Pat. No. 8,323,406, whichclaims the benefit of and priority to U.S. Provisional Application Ser.No. 60/880,869 filed Jan. 17, 2007, the disclosure of each of which ishereby incorporated by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with United States Government support under70NANB4H3051 awarded by the National Institute of Standards andTechnology (NIST). The United States Government has certain rights inthe invention.

FIELD OF THE INVENTION

The present invention relates to the fabrication of single crystal AlN,and, more specifically, to the fabrication of single crystal AlN havinglow planar defect densities.

BACKGROUND

Aluminum nitride (AlN) holds great promise as a semiconductor materialfor numerous applications, e.g., opto-electronic devices such asshort-wavelength light-emitting diodes (LEDs) and lasers, dielectriclayers in optical storage media, electronic substrates, and chipcarriers where high thermal conductivity is essential, among manyothers. In principle, the properties of AlN may allow light emissiondown to around 200 nanometers (nm) wavelength to be achieved. The use ofAlN substrates is also expected to improve high-power radio-frequency(rf) devices, made with nitride semiconductors, due to the high thermalconductivity with low electrical conductivity. Addressing variouschallenges can help increase the commercial practicability of suchdevices.

For example, large-diameter bulk AlN crystals (grown, for example, usingthe techniques described in U.S. application Ser. No. 11/503,660,incorporated herein in its entirety, referred to hereinafter as the“'660 application”), may in some circumstances grow with hexagonal-prismshaped cavities defects that are roughly 0.5 millimeter (mm) in diameterand 0.1 mm thick. Area concentrations as high as 100 cm⁻² have beenobserved in AlN slices that are cut to be 0.5 mm thick from these largediameter boules. Similar kinds of defects have been observed in thegrowth of other hexagonal crystals, such as SiC, and are commonlyreferred to as planar defects. These defects may be problematic for thefurther development of nitride-based electronics. In particular, theytypically cause the surface of a substrate to roughen when theyintersect the surface plane. They may also scatter light, which may beproblematic for many opto-electronic applications that benefit from thetransparency of AlN substrates at optical wavelengths between 210 and4500 nm. Planar defects may also reduce the thermal conductivity aroundthe defect, an effect that is generally undesirable for high-powerdevices in which the high intrinsic thermal conductivity of the AlN isuseful. They may also introduce small angle grain boundaries into theAlN crystal and, thus, degrade the quality of the crystal by increasingthe effective concentration of dislocations that thread through from oneside of the wafer to the other (so-called threading dislocations) andthat degrade the quality of surface preparation. Thus, the applicationof AlN substrates to the fabrication of high-performance, high-poweropto-electronic and electronic devices may be enhanced if planar defectsare reduced or eliminated.

Generally, planar defect formation in crystals grown by physical vaportransport (PVT) is caused by voids that get trapped in the growingcrystal and that move and are shaped by the thermal gradients to whichthe crystal is exposed. A common cause identified in SiC crystal growthis poor seed attachment, where any kind of a microscopic void willcommonly result in the formation of a planar defect (see, e.g., T. A.Kuhr, E. K. Sanchez, M. Skowronski, W. M. Vetter and M. Dudley, J. Appl.Phys. 89, 4625 (2001) (2001); and Y. I. Khlebnikov, R. V. Drachev, C. A.Rhodes, D. I. Cherednichenko, I. I. Khlebnikov and T. S. Sudarshan, Mat.Res. Soc. Proc. Vol. 640, p. H5.1.1 (MRS 2001), both articles beingincorporated herein by reference in their entireties). In particular,poor seed attachment may cause voiding to occur between the seed andseed holder or may leave the back surface of the seed inadequatelyprotected, allowing AlN material to sublime from that surface. For AlNcrystal growth, crucible abnormalities, such as wall porosity or a seedmounting platform in which voids are present or can form, may also be acause of voiding.

A typical planar defect 10 is shown schematically in FIG. 1. In somecases the shape of the planar defects is not perfectly hexagonal butmodified or distorted and even triangular depending on the tilt betweenthe planar void and the c-plane {0001} of AlN. In addition, there istypically a small-angle grain boundary 20 in the trail of the planardefect as shown in the schematic diagram, the origin of which isdiscussed below. The planar defect has a height h₁, and leaves a planardefect trail of length h₂ that extends back to the origin of the planardefect, typically the back of the seed crystal.

FIGS. 2 a and 2 b show optical microscopy images of a 2-inch diameter,c-face (i.e., c-axis oriented parallel to the surface normal of thewafer) AlN substrate taken after fine mechanical polishing. Theright-side image (FIG. 2 b) represents the same location as in theleft-side image (FIG. 2 a) taken in cross-sectioned analyzer-polarizer(AP) mode. The planar defect dimensions vary from 0.1 to 2 mm in widthand up to 0.5 mm in depth, although they generally tend to be thinner(˜0.1 mm). However, the base of the planar defect is typicallymisoriented with respect to the overall crystal (typically a smallrotation about the c-axis), and thus there is a boundary between theoriginal crystal and the slightly misoriented material that is below theplanar defect. This boundary is defined by dislocations that account forthe misorientation of the material below the planar defect.

Causes of Planar Defects

If the AlN seed is poorly attached in a way that allows material in theback of the seed to move under the temperature gradient, then thismaterial movement may cause voids to “enter” the seed. This effect isdue to the fact that every void has a small but defined axial gradientthat drives material to be evaporated and then re-condensed within thevoid. The voids entering the AlN bulk material form well-definedhexagonal-prism shapes, probably because of the anisotropy in surfaceenergy formation.

Migration of the Planar Defect in a Thermal Gradient and ResultingDegradation of Crystal

Referring to FIGS. 3 a and 3 b, growth inside planar defects has beendocumented. The growth facet in FIG. 3 b is pronounced, indicatingfaceted growth mode within the planar defect. Faceted growth modeusually results in a high-quality crystal. It can be expected,therefore, that the material quality within the planar defect is highand may be dislocation-free.

As the crystal grows, the planar defects effectively migrate toward thegrowth interface due to the axial temperature gradient within the void.Planar defects travel from the seed toward the growth interface becauseof the axial gradient across the planar height. As a result of thismovement, the planar defects may leave “trails” (or imprints) of grainboundaries with very small misorientation angles. These small-anglegrain boundaries are pronounced and shaped according to the planardefect symmetry. An example of this is shown in FIG. 4 and discussedbelow.

According to the traditional Read's model of low-angle grain boundaries,a boundary typically contains pure edge dislocations lying in the planeof the boundary. Therefore, after etching, the boundary is expected toexhibit a number of separated etch pits. The greater the distancebetween the pits, the smaller will be the misorientation angle. Thegrain boundary angle may be found using Frank's formula:

$\begin{matrix}{{\frac{b}{D} = {2{\sin\left( \frac{\theta}{2} \right)}}},} & (1)\end{matrix}$where D is the distance between dislocations (etch pits), b is theBurgers vector of dislocation, and θ is the misorientation angle. InFIG. 4, the closest distance between the etch pits is ˜12 micrometers(μm), and the Burgers vector for pure edge dislocation perpendicular tothe {0001} planes is equal to the “a” lattice constant, i.e. 0.3111 nm.Therefore, the azimuthal misorientation angle of the planar defect wallsis expected to be about 0.0004° (or 1.44 arcsec).

Thus, in addition to the problems caused by the physical presence ofplanar defects, the formation and motion of planar defects in thecrystal during growth may also degrade the overall crystal quality. Thisdegradation results because of the slight misorientation between theplanar defect body and the AlN bulk material. As the planar defect movesthrough the crystal, it leaves behind a grain boundary, as shown inFIG. 1. These grain boundaries typically show misorientation of about 2arcseconds for individual planar defects. However, if the density of theplanar defects is high, each of these randomly misoriented grainboundaries can add up and result in much higher “effective”misorientation and, as a result, much lower crystal quality. Analternative way to look at the degradation of crystal quality is toconsider the increase in threading dislocation density due to the planardefects. As one may calculate from the micrograph shown in FIG. 4, eachplanar defect may create over 10⁴ dislocations/cm² in its wake.

Problems with Surface Preparation Due to Planar Defects

Planar defects may affect preparation and polishing of AlN wafers. Thesharp edges of planar defects intersecting the AlN sample surface maychip off and cause scratching. In addition, planar defects—being relatedto the small-angle grain boundaries (SAGB)—may result in surfaceroughening (topography) during chemical-mechanical polishing (CMP)treatment.

FIG. 5 shows the surface and the bulk depth of AlN containing planardefects and LAGB, where the images are obtained at the same location. Itis clear that the planar defects and the SAGB cause surface rougheningwhich, in turn, affects the epitaxial growth.

Problems with Optical Transparency and Thermal Conductivity

Planar defects may have a negative impact on the optical-transmissionproperties of AlN wafers because they scatter light due to theintroduction of additional interfaces within the crystal, which separateregions with different refractive indices. In addition, while AlNsubstrates are attractive because of their high thermal conductivity(which can exceed 280 W/m-K at room temperature), planar defects maycause the thermal conductivity to diminish in a location directly abovethe planar defect because of the extra interfaces that are inserted atthe planar defect boundaries as well as the thermal resistance of thevolume of the planar defect itself. This local increase of the thermalresistance of the AlN substrate may reduce the usefulness of the AlNsubstrates for applications that require high power dissipation, e.g.,high-power RF amplifiers and high-power, high-brightness LEDs and laserdiodes.

Limitations of Existing Methods

As described in the '660 application, the production of large-diameter(i.e., greater than 20 mm) AlN crystals typically requires seededgrowth. However, as discussed below, the seed holder and seed mountingtechnique on the holder are primary sources of planar defects in the AlNboules that are produced. The '660 application discloses a method forAlN seed attachment and subsequent crystal growth. Referring to FIG. 6,an AlN ceramic-based, high-temperature adhesive bonds the AlN seed tothe holder plate and at the same time protects the back of the AlN seedfrom sublimation. In particular, an AlN seed 100 is mounted onto aholder plate 130 using an AlN-based adhesive 140. The AlN ceramicadhesive may contain at least 75% AlN ceramic and silicate solution thatprovides adhesive properties. One suitable example of such an adhesiveis Ceramabond-865 available from Aremco Products, Inc.

In a particular version, the AlN seed is mounted using the followingprocedure:

-   -   (1) AlN adhesive is mixed and applied to the holder plate using        a brush to a thickness not exceeding about 0.2 mm;    -   (2) The AlN seed is placed on the adhesive; and then    -   (3) The holder plate along with the seed is placed in a vacuum        chamber for about 12 hours and then heated up to 95° C. for        about 2 hours.

This approach has proven successful in providing high-quality,large-diameter AlN crystal boules. However, planar defects as shown inFIG. 2 will form. This problem is caused by the voids left behind as thesilicate solution is either evaporated or absorbed by the AlN seedcrystal or by Al escaping through the seed holder.

An alternative method for AlN seed attachment and subsequent crystalgrowth described in the '660 application involves mounting the AlN seedon a thin foil of Al on the holder plate. The Al is melted as thetemperature of the furnace is raised above 660° C. (the melting point ofAl), thereby wetting the back of the seed and the holder plate. As thetemperature is raised further, the Al reacts with N₂ in the furnace toform AlN, which secures the seed to the holder plate. This technique mayrequire that the AlN seed be held in place (either by gravity ormechanically) until a sufficient amount of the Al has reacted to formAlN, after which no further mechanical support is needed.

This technique, too, results in planar defects. The Al foil may melt andball up, leaving empty spaces between agglomerations of liquid Al. Theagglomerated liquid-Al metal may then react to form a nitride, leavingempty spaces between the seed and the seed holder. These empty spaces,in turn, can lead to planar defects once crystal growth is initiated onthe seed crystal. The interaction between the AlN seed and the seedholder may also contribute to defects. Typically some amount ofdiffusion of either Al or N (or both) into the seed holder occurs at thetemperatures used for crystal growth. For instance, a tungsten (W) seedholder may absorb both Al and N at the growth temperature, which canresult in planar defects forming in the seed crystal and in theresulting boule grown from the seed crystal. In addition, the seedholder may have a thermal expansion coefficient different from that ofthe AlN crystal, which may cause defects in the seeded crystal or mayinduce voids to open up at the seed crystal/seed holder interface,resulting in planar defects during subsequent boule growth.

Another way to attach the seed crystal to the seed holder is to run aheat cycle under conditions whereby the seed is held onto the seedbacking (e.g., by placing the seed crystal under an appropriate massthat holds the crystal down during this process), and heating thecrystal up to a temperature above 1800° C. (and preferably above 2000°C.) to allow the seed to thermally, chemically and/or mechanically bondto the seed holder material. This approach is referred to herein assinter bonding. The sintering process may, however, be difficult tocontrol such that good bonding occurs without damaging the seed. Inaddition, it may be difficult to avoid leaving some space between theseed crystal and the seed holder. This space may be filled duringprocessing with AlN that mostly comes from the seed crystal (even whenvapors of Al and N₂ are supplied by having an AlN ceramic present in thecrucible during the sintering process), and this AlN may induce planardefects to form in the seed crystal that may propagate into thesingle-crystal boule grown on the seed crystal.

SUMMARY

Embodiments of the invention allow the reduction or elimination ofplanar defects during the growth of bulk aluminum nitride (AlN)crystals, i.e., boules. In particular, in some embodiments, the arealplanar defect density is reduced to less than 100/cm² and, preferably,to less than 1/cm². As a result, the fabrication of crystalline AlNwafers larger than 20 mm in diameter with a thickness ranging from 0.1-1mm and having planar defect density of less than 1 cm⁻² is enabled.

Key factors that enable growing seeded, large diameter, high quality AlNcrystals include:

1.) The seed crystal itself is free of planar defects, as well as freefrom other kinds of defects that may form voids (that generally evolveinto planar defects at the crystal growth temperature). A defect to beconsidered is subsurface damage that may be introduced into the seedcrystal by the cutting and polishing process.

2.) The seed is attached to a seed holder (with seed holder beingdefined in the '660 application and described in detail above withreference to FIG. 6) in such a way as to prevent the formation of voidsbetween the seed and the seed holder This may be accomplished byproperly preparing the back surface of the seed (also referred to as themounting surface of the seed, as opposed to the front surface of theseed that is used to seed the bulk crystal growth) as well as thesurface of the seed holder. A film is then applied to the back of theseed, conforming microscopically with both the back surface of the seedas well as the seed holder. This film is preferably completely dense(i.e., no microscopic voids).

3.) The seed holder is relatively impervious to aluminum transport so asnot to form voids in growing AlN crystal. In some examples, the filmused to attach the seed to the seed holder is, itself, impervious toaluminum transport. In some of the implementations described below, theseed holder is only impervious to aluminum transport for a certainperiod of time. This time limitation generally limits the length of theAlN boule that can be grown.

Since a principal way the AlN crystal compensates for the diffusion ofAl out of the crystal is by the formation of planar defects, the maximumallowable rate for transport through the seed holder assembly may beestimated for a given planar defect density. For instance, to keep thedensity of planar defects below 100/cm², the maximum allowable number ofatoms of Al that may be allowed to diffuse through the seed holderassembly is generally <10²⁰/cm² over the period of time that the AlNcrystal is being grown. To keep the planar defect density below 1/cm²,the Al diffusion is preferably kept below 10¹⁸ atoms of Al per cm².

4.) Stress between the seed holder assembly and the seed is reduced.This may be achieved by (i) either the thermal expansion of the seedholder assembly nearly matching the thermal expansion of the AlN seed inthe temperature range from room temperature up to the growth temperature(˜2200° C.), or (ii) the seed holder assembly being sufficientlymechanically flexible to absorb the thermal expansion mismatch throughdeformation while reducing the strain on the seed crystal and theresulting AlN boule. This factor does not typically allow theachievement of the third factor above by simply making the seed holderthicker.

5.) Generally, it is also desirable for the seed holder assembly to haveenough mechanical strength to be able to support the growing AlN boulewhile, at the same time, providing a sealing surface to the crucibleused to contain the AlN material and Al vapor (as described in the '660application). However, the mechanical strength needed typically dependson the crystal growth geometry used. Less mechanical strength may beneeded if the seed crystal is placed at the bottom of the crystal growthcrucible; this geometry, however, may need tighter control of the AlNsource material to prevent particles falling from the source materialnucleating defects in the growing crystal.

Moreover, conditions for high quality AlN crystal growth are preferablyfollowed, as described in the '660 application. In particular,super-atmospheric pressures may be successfully utilized to producesingle crystals of AlN at relatively high growth rates and crystalquality. To achieve this, one or more of the following may becontrolled: (i) temperature difference between an AlN source materialand growing crystal surface, (ii) distance between the source materialand the growing crystal surface, and (iii) ratio of N₂ to Al partialvapor pressures. Increasing the N₂ pressure beyond the stoichiometricpressure may force the crystal to grow at a relatively high rate due tothe increased reaction rate at the interface between the growing crystaland the vapor. This increase in the growth rate has been shown tocontinue with increasing N₂ partial pressure until diffusion of Al fromthe source to the growing crystal (i.e., the negative effects ofrequiring the Al species to diffuse through the N₂ gas) becomes therate-limiting step. Employing higher-pressure nitrogen may have theadded benefit of reducing the partial pressure of aluminum inside thegrowth crucible, which may decrease corrosion within the furnace oftencaused by Al vapor inadvertently escaping the crucible. To growhigh-quality AlN crystals, very high temperatures, for example exceeding2100° C., are generally desirable. At the same time, high thermalgradients are needed to provide sufficient mass transport from thesource material to the seed crystal. If not chosen properly, thesegrowth conditions may result in evaporation of seed material or itstotal destruction and loss. The AlN seeded bulk crystal growth may becarried out in a tungsten crucible using a high-purity AlN source. Thetungsten crucible is placed into an inductively heated furnace so thatthe temperature gradient between the source and the seed material drivesvapor species to move from hotter high purity AlN ceramic source to thecooler seed crystal. The temperature at the seed interface and thetemperature gradients are monitored and carefully adjusted, ifnecessary, in order to nucleate high-quality mono-crystalline materialon the seed and not destroy the AlN seed.

Hereinafter, several ways to implement these concepts are described indetail, and specific examples of implementation are provided.

In an aspect, embodiments of the invention may include a bulk singlecrystal of AlN having a diameter greater than 20 mm, a thickness greaterthan 0.1 mm, and an areal planar defect density may be less than orequal to 100 cm⁻².

One or more of the following features may be included. The areal planardefect density may be measured by counting all planar defects in thebulk single crystal and dividing by a cross-sectional area of the bulksingle crystal disposed in a plane perpendicular to a growth directionthereof. The bulk single crystal may be in the form of a boule having athickness greater than 5 mm. The areal planar defect density may be lessthan or equal to 1 cm⁻².

The single crystal AlN may be in the form of a wafer. The areal planardefect density may be less than or equal to 10 cm⁻². An areal planardefect density of planar defects intersecting each of a top and a bottomsurface of the wafer may be less than or equal to 1 cm⁻².

In another aspect, embodiments of the invention may include a bouleincluding a bulk single crystal of AlN having a diameter greater than 20mm, a thickness greater than 5 mm, and an areal density of threadingdislocations of less than or equal to 10⁶ cm⁻² in each cross-section ofthe bulk single crystal disposed in a plane perpendicular to a growthdirection of the crystal. In some embodiments, the areal density ofthreading dislocations may be less than or equal to 10⁴ cm⁻².

In yet another aspect, embodiments of the invention feature a bouleincludes a bulk single crystal of AlN having a sufficient thickness toenable the formation of at least five wafers therefrom, each waferhaving a thickness of at least 0.1 mm, a diameter of at least 20 mm, anda threading dislocation density of less than or equal to 10⁶ cm⁻². Insome embodiments, each wafer may have a threading dislocation densityless than or equal to 10⁴ cm⁻².

In still another aspect, embodiments of the invention include a bouleincluding a substantially cylindrical bulk single crystal of AlN havinga diameter of at least 20 mm and having a sufficient thickness to enablethe formation of at least five wafers therefrom, each wafer having athickness of at least 0.1 mm, a diameter of at least 20 mm, and atriple-crystal X-ray rocking curve of less than 50 arcsec full width athalf maximum (FWHM) for a (0002) reflection. Each wafer hassubstantially the same diameter as each of the other wafers.

In another aspect, embodiments of the invention include a method forgrowing single-crystal aluminum nitride (AlN). The method includesproviding a holder including a backing plate, the holder (i) being sizedand shaped to receive an AlN seed therein and (ii) including an AlNfoundation bonded to the backing plate. An Al foil is interposed betweenthe seed and the AlN foundation. The Al foil is melted to uniformly wetthe foundation with a layer of Al. An AlN seed is disposed within theholder. Aluminum and nitrogen are deposited onto the seed underconditions suitable for growing single-crystal AlN originating at theseed.

One or more of the following features may be included. The back platemay be conditioned to reduce permeability of the back plate to Al. Theseed crystal may be a wafer having a diameter of at least 20 mm. Thegrown single-crystal AlN may define a boule having a diameterapproximately the same as a diameter of the seed crystal.

In another aspect, embodiments of the invention feature a method forgrowing single-crystal aluminum nitride (AlN). The method includesproviding a holder sized and shaped to receive an AlN seed therein, theholder consisting essentially of a substantially impervious backingplate. An AlN seed is disposed within the holder. An Al foil isinterposed between the seed and the backing plate. The Al foil is meltedto uniformly wet the backing plate and the back of the AlN seed with alayer of Al. Aluminum and nitrogen are deposited onto the seed underconditions suitable for growing single-crystal AlN originating at theseed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the samefeatures throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic diagram showing an idealized planar defect whichtypically appear as hexagonal voids in the AlN crystal;

FIGS. 2 a and 2 b are optical micrographs of AlN single crystal samplecontaining planar defects: a) an optical image showing one planardefect, and b) image taken from the same location in birefringencecontrast showing multiple planar defects just underneath the surface;

FIGS. 3 a and 3 b are micrographs demonstrating growth features insideplanar defects, that are due to the movement of the planar defectsduring the crystal growth, with FIG. 3 a being an optical micrograph,and 3 b being a micrograph taken in Nomarski Differential Image Contrast(NDIC);

FIG. 4 is an NDIC micrograph of the planar defect wake and grainboundaries marked by an etch pit associated with dislocations;

FIGS. 5 a and 5 b are micrographs illustrating the effect of thelow-angle grain boundaries due to planar defects on the surface finish,with FIG. 6 a taken after the CMP process, and 6 b being a birefringencecontrast image from the same location showing planar defects;

FIG. 6 is a schematic diagram of an AlN seed mounting technique usinghigh temperature AlN ceramic-based adhesive;

FIG. 7 is a graph illustrating the axial distribution (along the growthaxis) of the density of planar defects intersecting the surface of aslice through the boule: wafer #1 is closest to the growth interfacewhile wafer #4 is closest to the seed;

FIG. 8 is a schematic diagram illustrating the bonding of an AlN seed toa seed holder, (which, in a preferred implementation, consists of an AlNfoundation on a W backing plate);

FIG. 9 is a schematic diagram illustrating the technique for bonding anAlN seed crystal to a seed holder, which, in a preferred implementation,uses Al-foil nitridation; and

FIG. 10 is a schematic diagram illustrating an assembled crystal growthcrucible.

DETAILED DESCRIPTION

In accordance with some embodiments of the invention, one or more of themeasures described below may be taken to reduce defect generation duringseeded AlN growth of, e.g., boules.

As used herein, boule means an as-grown crystal of AlN that haspredominately (more than 50%) a single orientation. To betechnologically useful, the boule is preferably at least 20 mm indiameter and more than 5 mm in length, and the orientation preferablyvaries by no more than 1.5° across the width of the boule.

As used herein, wafer means a slice of AlN cut from a boule. Typically,a wafer has a thickness of between 0.1 mm to 1 mm and a diameter greaterthan 20 mm. However, wafers thinner than 0.1 mm, while fragile, may betechnologically useful for some specialized applications (for instance,in an application where optical transmission through the wafer iscritical).

High quality bulk single crystal AlN having low planar defect densities,and methods for formation thereof, are disclosed herein. Referring againto FIG. 4, each planar defect may create over 10⁴ dislocations/cm² inits wake. Thus, to prepare AlN wafers (which are thin slabs, typically0.1 to 1 mm thick, cut from the bulk crystal) with threading dislocationdensities (TDD) below 10⁶/cm², the areal planar defect density (definedas the number of planar defects that have passed through a unit area inthe bulk crystal) is generally kept below 100/cm² or below 1/cm² if theTDD is to be kept below 10⁴/cm². An areal planar defect density may bemeasured by counting all planar defects in a bulk single crystal anddividing by a cross-sectional area of the bulk single crystal disposedin a plane perpendicular to a growth direction thereof. Because thetemperature gradient along the crystal height increases toward the seed,so one may generally expect that the density of planar defects woulddecrease toward the growth interface (crown).

An illustration of this effect in AlN boules is shown in FIG. 7, thatshows the axial distribution (along the growth axis) of the density ofplanar defects that intersect the surface of the wafer, with wafer #1being closest to the growth interface (crown) while wafer #4 is closestto the seed. The planar defects that intersect the surface of wafer #1have passed through the region of the boule represented by wafer #4,which may be observed by etching their trails as shown in FIG. 4. Sinceit may be difficult to see all the planar defects in a thick boule, theareal density may be measured by cutting a thin slice (0.1 to 0.8 mmthick) from the boule perpendicular to the growth direction andpolishing both surfaces of the slice with any anisotropic etch. Theareal planar defect density may then be estimated by totaling the numberof planar defects observed in the slice (both on the surface and underthe surface) and the number of planar defect trails that are observed onthe surface of the slice due to the preferential etching of defects, andthen dividing by the area of the slice. As FIG. 7 shows, the arealdensity of planar defects measured in a slice near the original seedcrystal will generally be higher than the areal density measured nearthe crown. Thus, to get the true areal planar defect density (and, thus,determine the number of low defect wafers that can be sliced from theboule) a slice from the boule is preferably selected from near the seedside of the boule. The areal densities of planar defects in wafers orseed plates sliced from a boule may be measured in the same way.High-resolution x-ray diffraction (XRD) rocking curves are a commonlyused indication of the crystal quality and may be used to estimate thedislocation density. See Lee et al., “Effect of threading dislocationson the Bragg peakwidths of GaN, AlGaN, and AlN heterolayers,” Appl.Phys. Lett. 86, 241904 (2005), incorporated by reference in itsentirety. Based on this paper, it can be estimated that to obtain lessthan 50 arcsec full width at half maximum (FWHM) for a triple-crystalx-ray rocking curve of the (0002) reflection (for a c-face wafer), theareal planar defect density is preferably below 100/cm².

The yield from a boule (the number of wafers that can be sliced from theboule that meet the size and defect specification) may be increased byreducing the areal density of planar defects in the boule and byincreasing the length of the boule. Preferably, a technologically usefulboule yields at least 5 wafers that meet the size and defectspecifications.

1. Preparation of the Seed Crystal

In the implementations discussed below, a high quality AlN seed crystalis prepared. The AlN seed crystal is preferably cut from asingle-crystal boule grown as described herein (i.e., a portion or allof a resultant boule is used to form seed plates for subsequent crystalgrowth). Typically the seed crystals are cut as round plates of about 2inches (50-60 mm) in diameter and having a thickness ranging from 0.2 upto 5.0 mm. However, smaller area seeds may also be prepared to be ableto select seeds formed from very high quality regions of a boule ofnonuniform quality or because a different crystal orientation isdesired. These smaller diameter seeds may be mined from AlN crystalboules grown as described herein. Seed plates or smaller area seedcrystals may also be prepared by slicing AlN boules fabricated by othertechniques, such as the technique described in the '660 applicationwhere a high quality encased AlN seed crystal, formed by selfnucleation, is used to seed the AlN crystal growth and the crystalgrowth crucible is arranged so as to expand the diameter of theresultant AlN boule up to 2 inches in diameter, as shown in FIG. 7 ofthat application. In all cases, it is important that high quality,nearly defect free seeds be selected, because defects in the seedcrystal(s) may be duplicated in the AlN boule to be produced. Inparticular, the areal density of planar defects in the seed crystals ispreferably below 100 cm⁻² and, even more preferably, below 1 cm⁻². Ifmultiple small area seeds are to be used simultaneously, the orientationof each seed is preferably carefully controlled so that the seeds can bematched when they are mounted on the seed holder.

The orientation of the seed crystal plate (or of the smaller seedcrystals) is typically with the c-axis parallel to the surface normal ofthe plate (a so-called c-axis seed plate), but other orientations andsizes are suitable as well. The surface of the AlN seed crystal thatwill face the seed holder assembly (the seed back side) is preferablysmooth and flat with a total thickness variance (TTV) of less than 5 μmand preferably less than 1 μm so that gaps between the seed crystal andthe seed holder assembly are reduced. A “smooth surface,” as usedherein, is a surface that has no scratches visible when viewed with anoptical microscope under 200× magnification and that the root meansquare (RMS) roughness measured with an atomic force microscope (AFM) isless than 1 nm in a 10×10 μm square area. Optical measurement techniquesare effective for measuring the TTV.

The top surface of the AlN seed crystal (which will serve as thenucleation site of the AlN crystal boule) is preferably smooth. Inaddition, any crystal damage in the top surface of the AlN seed crystalthat may have resulted from cutting or polishing the seed crystal ispreferably removed prior to attaching the seed crystal to the seedholder. This subsurface damage (SSD) layer may be removed in accordancewith methods described in U.S. Ser. No. 11/363,816 (referred tohereinafter as the “'816 application”) and Ser. No. 11/448,595 (referredto hereinafter as the “'595 application”), both of which areincorporated herein by reference in their entireties. An exemplarymethod includes performing a CMP step by applying an abrasive suspensionin a solution consisting essentially of a hydroxide. Another exemplarymethod is a CMP process that includes polishing a substrate using aslurry including an abrasive suspension in a solution capable ofmodifying the surface material of the substrate and creating a finishedsurface suitable for epitaxial growth. The active solution chemicallymodifies the surface of the substrate, forming a compound softer thanthe underlying substrate material. The abrasive is selected to be harderthan the newly created compound, but softer than the substrate material,so that it polishes away the newly formed layer, while leaving thenative substrate surface pristine and highly polished.

The specific recipe for SSD removal depends on the seed orientation.Removal of the SSD layer is important as it preferentially thermallyetches, leaving void and defect spaces as well as irregular topographyat the interface between the seed crystal and the resulting AlN boulethat may compromise crystal growth and may result in planar defects. Inparticular, improvements in polishing of the seed crystal may improvethe quality of the boule growth by reducing defects during thermalcycling. A suitable seed will have planar and/or extended voids of lessthan 1 per square centimeter intersecting either surface of the seed,less than one scratch of 10 nm depth within a 10×10 μm square AFM scanand less than one crack per cm².

Other defects that are preferably avoided include pits, grain boundaries(including polarity inversions) and cracks. In addition, surfacecontamination due to, for instance, polishing, handling, and oxidation,is undesirable. Void formation from the inclusion of scratched materialis a risk. Areas with SSD are more likely to thermally etch during theseed mounting heating cycle. Thermal etching of the AlN seed crystal orbacking material may create a void space. In addition, SSD representsdamaged crystal lattice within the seed crystal. Defective crystallattice within the seed crystal is generally replicated within the grownboule and may lead to the creation lower quality wafers that are cutfrom that boule. Thermal etching of the seed crystal may be mitigated byusing a lower mounting temperature (lower mounting temperature mayreduce thermal etching) or by gas species/pressure choices (highpressure N₂/argon/xenon, etc. may suppress thermal etching) but mayleave SSD that will be replicated in the seeded growth.

Voids present in the seed material may create voids in the grown boule.Voids intersecting the back surface of the seed may lead to seedmounting difficulties. Voids intersecting either the seed holder orgrowth interface surface of the seed may present contamination issues(trapped material). Therefore, seeds for seeded growth desirably areeither cut from boules that have been grown by these void-free methodsor cut from AlN boules generated by self-nucleation techniques describedin the '660 application.

In particular, as discussed in the '660 application, two conditions maybe considered to employ self-nucleation in the preparation of AlNboules. First, there is a nucleation barrier to the growth of AlN ontungsten. That is, the vapor above a tungsten crucible tends to besupersaturated unless AlN nuclei are available for growth. To takeadvantage of this, a seeded region may take up some part of the fulldiameter seed mounting plate that is surrounded by an unseeded, bareregion. Since adsorption of aluminum and nitrogen from the vapor ontothe seed is favored over deposition onto the bare crucible wall, theseed is favored to expand laterally in favor of creating new self-seededcritical nuclei next to the seed. Under properly controlled conditionsthis process can be used to increase the seeded area per growth cycle.Secondly, the process of crystal growth requires heat extraction whichis controlled by arrangements of insulators/heaters in the system.Properly arranging insulation so that the seed is the coolest part ofthe upper crucible and cooler than the source during growth is importantto the process. Further tailoring this insulation when using a smallseed to be expanded during the growth aids in expansion of the seed bymaking the seed cooler than the unseeded lateral region. This thermalarrangement makes self-seeded nucleations neighboring the seed lessfavored by limiting heat extraction. As the crystal grows at hightemperature and with sufficient source material, given sufficient timeto reach an equilibrium point during the growth run the interface of thecrystal will follow the isotherms of the system (insulation/heaters,etc). The proper interface shape to favor seed expansion is slightlyconvex in the growth direction; the curvature of the gradient aidsexpansion.

Residual SSD may be identified and other defects such as threadingdislocations (TDD) may be revealed with a defect etch using a KOHvapor/solution or with a KOH-enhanced CMP, as described in Bondokov etal. in “Fabrication and Characterization of 2-inch Diameter AlNSingle-Crystal Wafers Cut From Bulk Crystals” [Mater. Res. Soc. Symp.Proc. Vol. 955 (Materials Research Society, Pittsburgh, 2007) p.0955-103-08]. The density of the pits measured in these defect etches isreferred to as etch pit density (EPD). For seeded growth, it isgenerally desirable to start with seeds that have less then 10⁴ EPD. Itis possible to improve grown boule over seed quality, but it ispreferable to start with high quality seeds. It is also important toavoid cracking the seed.

2. Detailed Example of a Seed Crystal Preparation

The procedure used to prepare the seed crystal surface depends on itscrystallographic orientation, as described in the '816 application and'595 application. Briefly, as described in those applications,crystallographic orientation affects mechanical preparation of asubstrate surface prior to CMP processing; substantial differences existfor optimal substrate preparation. For example, in the case of an AlNsubstrate, the Al-terminated c-face is not reactive with water, but theN-terminated c-face is reactive with water, along with non-polar faces.During wet lapping and polishing, the Al-polarity face tends to chipunder the same conditions that are well-suited to mechanically polishthe non-Al-polarity faces or Al-polarity faces where the c-axis isoriented 20 degrees or more away from the surface normal of thesubstrate.

Here, we describe an exemplary process for preparing a c-axis seed platewhere the nitrogen-polarity face (N-face) will be attached to the seedholder assembly and the aluminum-polarity face (Al-face) will be used tonucleate the AlN boule. After an appropriately oriented seed plate iscut from an AlN boule using a diamond wire saw (the seed plate is cutsuch that the c-axis is within 5° of the surface normal), the surfacesare ground flat and then diamond slurries (with progressively decreasingdiamond size) are used successively to further mechanically polish bothsurfaces of the seed plate. More specifically, the N-face of theas-sliced AlN wafers undergoes grinding (with 600 diamond grit), lapping(6 μm diamond slurry), and fine mechanical polishing with 1 μm diamondslurry. Then, the wafer is flipped over and the Al-face undergoesgrinding (with 600 and 1800 diamond grit), lapping (6 μm and 3 μmdiamond slurries), and fine mechanical polishing with 1 μm diamondslurry followed by the CMP, as described in the '816 application where ahigh pH silica suspension in a KOH solution is used to leave anAl-polarity, c-face surface that is free of SSD.

These mechanical polishing steps may be followed by a CMP step on theN-face of the seed crystal (which is the back surface that will bemounted facing the seed holder assembly in this example). A suitableslurry is a 1 μm Al₂O₃ slurry with active chemical solution (the slurryis made of 100 grams of 1 μm Al₂O₃ grit per 1 liter of solution composedof 0.5M KOH in distilled water (1 liter) with an additional 50 mL ofethylene glycol). The slurry is used on a soft composite iron polishingdeck such as the AXO5 from Lapmaster, Inc.), leaving the surface highlyreflective to the eye and free of defects such as scratches or pits oropen cracks. The grit choice and active chemical reaction between theAlN and the strong base (KOH) are important for producing a surface withlow defect densities. A preferred surface has less than 1 scratch deeperthan 10 nm per 10 square μm scan with an AFM and the RMS roughnessmeasured with an AFM is less than 1 nm in a 10×10 μm area. In addition,the back side of the seed crystal surface preferably has a TTV of lessthan 5 μm and more preferably less than 1 μm. This is important becausesurface topography, even at a microscopic level, may result in planardefects forming in the seed crystal; these defects may propagate intothe crystal boule during subsequent growth. The flatness of the polishedsurfaces is checked using a suitable optical flat and monochromaticlight source (sodium lamp at 590 nm).

The Al-face is then subjected to a final CMP step after the 1 μm diamondpolishing step using a silica suspension from Cabot Industries (Cabot43). Additional techniques for preparing the surface of the seedcrystals are described in the '816 application and the '595 application.For example, as noted above, the CMP process may include polishing asubstrate using a slurry including an abrasive suspension in a solutioncapable of modifying the surface material of the substrate and creatinga finished surface suitable for epitaxial growth. The active solutionmay modify the surface of the substrate, forming a compound that issofter than the underlying substrate material. The abrasive may beselected to be harder than the newly created compound, but softer thanthe substrate material, so that it polishes away the newly formed layer,while leaving the native substrate surface pristine and highly polished.In some CMP processes, the slurry may include an abrasive suspension ina solution consisting essentially of a hydroxide.

The seed crystal is now ready for mounting on one of the seed mountingassemblies described below and is preferably carefully stored in anitrogen atmosphere glove box to avoid any contamination prior togrowth.

3. Seed Holder Plates

Different structures have been developed for the seed holder plate. Thepreferred approach depends on the particular circumstances used forcrystal growth.

3.1. Textured AlN Deposited on a Backing Plate

Referring to FIG. 8, in an embodiment, a seed holder 800 may include arelatively thick, highly textured AlN layer, i.e., foundation 810,deposited on a metal backing plate 820, e.g., W foil. The holder 800 issized and shaped to receive an AlN seed therein. Preparation of apreferred embodiment may include one or more of the following threefeatures:

-   -   a.) The use of a seed holder including an AlN foundation bonded        to an appropriate backing plate (in a preferred embodiment, this        backing plate is W foil);    -   b.) Appropriately conditioning the backing plate so that it is        nearly impervious to Al diffusion through the plate; and/or    -   c.) Using Al foil to form an adhesive 140 to bond the seed to        the AlN ceramic or seed plate by heating the seed plate/Al        foil/AlN seed crystal to high temperature rapidly enough so that        the Al first melts and uniformly wets the AlN with a very thin        layer of Al before converting into AlN.

In an embodiment, the W foil has a thickness of 20 mils to 5 mils (510to 130 μm). A thinner W foil may be desirable to reduce the stress thatthe seed plate will apply to the seed crystal and the resulting bouledue to the thermal expansion mismatch between the AlN and seed mountingW plate. The thickness of foil used for the mounting plate may be chosensuch that the specific vendor/lot of W-foil provides a relativelyimpervious barrier to aluminum and/or nitrogen. This W-backing orbarrier layer is preferably made from high density material (fortungsten>98% theoretical density) and may be made of multiple layers ofgrains allowing grain swelling to close fast diffusion paths betweengrain boundaries. The latter approach has also been described in U.S.patent application Ser. No. 11/728,027 (referred to hereinafter as the“'027 application”), incorporated herein by reference in its entirety.As discussed therein, machining of powder metallurgy bars includingtungsten grains having substantially no columnar grain structure is anexemplary method of forming multilayered and/or three-dimensionalnominally random tungsten grain structures that can help preventpermeation of aluminum through the tungsten material. In addition, thisW backing plate may be made from single crystal tungsten, that may nothave any grain boundary diffusion.

The W foil is preferably cleaned and conditioned with aluminum prior tocrystal growth. The foil may be further conditioned by applyingadditives such as Pt, V, Pd, Mo, Re, Hf, or Ta. Thicker layers oftungsten may be used to limit Al diffusion through the backing plate butthey will suffer from increased thermal expansion mismatch between thematerials leading to higher cracking densities in the grown AlNcrystals.

The polycrystalline W foil is preferably made of layers of grains. Thesestacked and compressed pure W-grains contain path ways between thegrains (where the grains meet neighboring grains) that allow diffusionpaths between the grains. Loss of aluminum is primarily through thesegrain boundaries and leads to voids (planar or extended) in the AlN. Intime, as these W grains absorb Al atoms through diffusion into the Wgrains, the W grains will swell as much as 5%, as Al is a substitutionalimpurity in W and has approximately a 5% solubility. As detailed in the'027 application, these swollen grains will decrease the grain boundarydiffusion rate. The Al-conditioning may be achieved at growthtemperature by processing similarly to the described AlN-foundationprocess. Rather than using Al to condition the W foil, other materialssuch as Pt, V, Pd, Mo, Re, Hf, or Ta may be used to decrease the amountof Al lost through grain boundaries by swelling, filling or decreasingthe grain boundary density in the W backing plate.

In the cases of Pt, V, or Pd, the elements may be applied (painted,sputtered, plated or added as foils) to the W foil and run through aheating cycle, preferably above the melting point of the added materialbut below the melting point of the tungsten, to allow the added elementto melt, leading to reaction with the W grains. This tends to cause theW grains to swell and to decrease both the time and Al required tofurther swell the grains and reduce Al losses through grain boundarydiffusion.

In the cases of Mo and Re, the elements may be mixed with the W to forman alloy. These alloys have a lower eutectic point with the Al presentunder growth conditions. This means that backing material composed ofthese alloys may not be suitable at as high a growth temperature as puretungsten. The lower eutectic point means that exaggerated grain growthtends to be faster than pure W with the same Al exposure conditions.While care must be taken to ensure that there are enough layers ofgrains in these alloy foils, the surface layers of grains will quicklyswell on exposure to Al vapor, which will prevent further Al diffusionalong the their grain boundaries. An additional advantage of Mo and Realloys with tungsten is that these alloys may have a smaller thermalexpansion mismatch with AlN, which will improve the cracking yield(i.e., fewer boules will be cracked).

In the cases of Hf and Ta, the applied layers on the W-foil may bereacted to form additional film or barrier layers on the W foil whichwill help to fill the grain boundaries in the W foil. The Hf or Ta canbe applied to the W-foil surface by adding powder, foil, sputtering orplating. The pure element spread over the polycrystalline W foil canthen be reacted with nitrogen or carbon to form HfN, HfC, TaC or TaNwhich will aid in sealing grain boundaries and will reduce the grainboundary diffusion rate through the W foil. These nitride or carbidecompounds could be applied directly as well provided they could beapplied in continuous layers forming a minimum of additional pathways orgrain boundaries through the layer.

Referring to FIG. 9, a single-crystal seed 100 is attached to a seedholder 800 using a weight 900. The single-crystal seed 100 attached tothe seed holder 800 by, e.g., adhesive 140.

The important elements of this approach are: (i) that the AlNfoundation, if properly formed, provides a nearly perfect thermalexpansion match to the growing AlN boule as well as an excellentchemical match; (ii) the backing plate, when properly conditioned,provides a nearly impervious barrier to Al diffusion; and (iii) therapid thermal processing of the Al foil along with excellent polishingof the AlN seed and the AlN foundation, provides a tight and dense bondbetween the foundation and the seed that will help prevent planardefects from forming.

4. Preferred Implementations

In a preferred embodiment, a polycrystalline AlN foundation is producedby the sublimation-recondensation technique described in the '660application, in which a relatively thick (3 to 5 mm) layer of AlNmaterial is deposited directly onto a metal foil or plate. The processincludes sublimation, wherein the source vapor is produced at least inpart when crystalline solids of AlN or other solids or liquidscontaining AlN, Al, or N sublime preferentially. The source vaporrecondenses on a growing seed crystal. It may be desirable to have thethickness of the AlN deposit be more than 10 times the thickness of themetal plate so that the relative stiffness of the AlN layersubstantially exceeds that of the metal plate. In this manner, themajority of the strain from any thermal expansion mismatch between themetal plate and the AlN foundation plus seed crystal (plus crystal bouleafter growth) may be taken up by the metal plate. It may be desirable tonot have the thickness of the foundation layer be too large, because agreater thickness may limit the size of the eventual crystal boule to begrown. For this reason, the thickness is preferably limited to less than20 mm. We have found that deposition of the AlN under typical growthconditions described in the '660 application can result in a highlytextured AlN film. In this context, a textured film means that almostall of the AlN grows in the form of grains having a c-axis (the [0001]direction using standard notation for hexagonal crystals) orientedparallel to the surface normal of the growing film. The diameter of thegrains in the plane perpendicular to the growth direction (i.e.,perpendicular to the [0001] crystallographic direction) is typically 0.1to ˜2 mm in size. An advantage of this highly textured film is itsbeneficial impact, derived from the fact that AlN has various thermalexpansion coefficients that depend on the crystallographic direction. Apolycrystalline film where the individual grains were randomly orientedmay crack as it is cycled from the growth temperature of approximately2200° C. to room temperature.

While the AlN is being deposited on the W foil, the surface of the Wfoil may become saturated with Al which, we have observed, will greatlyreduce further diffusion of Al through the foil. This phenomenon isdescribed in '027 application, where it is noted that the penetrationrate of aluminum along grain boundaries is reduced after the tungstengrains have swelled due to uptake of Al by bulk in-diffusion. It may bedesirable to form the polycrystalline W foil so that it containsmultiple layers of W grains. We have found that W foil that is 0.020 to0.005 inch thick (e.g., material supplied by Schwarzkopf, HC Starck, HCross) is satisfactory for this purpose. Other metal foils or plates arealso suitable; these include Hf, HfN, HfC, W—Re(<25%), W—Mo(<10%),pyrolitic-BN (also called CVD-BN), Ta, TaC, TaN, Ta₂N, carbon (vitreous,glassy, CVD, or POCO) and carbon coated with Ta/TaC, Hf/HfC and BN. Wehave also found it helpful (depending on the grain structure of thefoil) to precondition the W foil by exposing it to Al vapor and lettingthe surface layer saturate with Al prior to significant deposition ofthe AlN layer on top of the foil.

Following growth and cool down of the polycrystalline AlN layer on thebacking material (or as-grown foundation), the foundation may beinspected to determine the suitability of the as-grown foundation forfurther use in seed mounting. In some embodiments, suitable AlNfoundations exhibit no or low cracking (<1 crack per square cm), no orlow planar voiding (<1 planar intersecting the surface per square cm),and no or low areas of thin AlN deposition (sufficient grown thicknessto polishing to specification). Inclusion of cracks, voids or thinlayers behind the seed mount area may create void space behind the seedcrystal. This void space may migrate, as described previously, todeteriorate the seed crystal and grown AlN boule.

In the described configuration, the AlN foundation layer may act toreduce the forces from thermal expansion mismatch on the grown boule bymatching the thermal contraction of the grown AlN boule. The holderplate (backing layer of W-foil) acts as the layer that is relativelyimpervious to aluminum and/or nitrogen barrier layer preventingmigration of the crystal material leading to void formation.

After the AlN layer is deposited as described above, it is preferablypolished to a smooth and flat surface. As mentioned above, a “smoothsurface” in this context means that there are no visible scratches in anoptical microscope (200× magnification) and that the root mean square(RMS) roughness measured with an atomic force microscope (AFM) is lessthan 1 nm in a 10×10 μm area. This is important as surface topography,even at a microscopic level, may result in planar defects forming in theseed crystal; these defects may propagate into the crystal boule duringsubsequent growth. The flatness of the polished foundation surface maybe checked using a suitable optical flat and monochromatic light source(sodium lamp at 590 nm is typical). The foundation surface is preferablyflat across the seed area to better than 5 μm and preferably better than1 μm. The as-grown AlN foundation layer on the W seed backing foil ispolished in the manner of a fine mechanical preparation of asingle-crystal AlN substrate, e.g., as described in '816 application. Inan exemplary CMP process, substrate may be polished with a slurryincluding an abrasive suspension in a solution, such that the slurry iscapable of etching the substrate surface and creating a finished surfacesuitable for epitaxial growth. A silica suspension in a hydroxidesolution may be used, e.g., the KOH-based CMP slurry known in the art asSS25 (Semi-Sperse 25) available from Cabot Microelectronics or the Sytonslurry available from Monsanto. The W foil backing side of the AlN/Wfoundation (as grown) may be mounted to a polishing fixture using asuitable mounting adhesive (e.g., Veltech'sValtron—AD4010-A/AD4015-B—50CC thermal epoxy). The rough shape of thecomposite may be leveled by polishing the AlN layer using a roughmechanical step. A suitable approach is to use a 15 μm diamond slurry ona steel polishing deck (e.g., a Lapmaster 12″ or an Engis LM15 with aregular steel deck). This rough mechanical step may be followed by afine mechanical process with 1 μm Al₂O₃ slurry in a KOH solution (theslurry is made of 100 grams of 1 μm Al₂O₃ grit per 1 liter of solutionwhich is composed of 0.5M KOH in distilled water (1 liter) with anadditional 50 mL of ethylene glycol). The composite is polished withthis slurry on a soft composite iron polishing deck such as the AXO5from Lapmaster, Inc.), leaving the surface highly reflective to the eyeand free of defects such as scratches or pits or open cracks. The gritchoice and active chemical reaction between the AlN and the strong base(KOH) is important to produce a surface with low defects. The preferredsurface has less than 1 scratch deeper than 10 nm per square 10 μm scan(AFM) and TTV of less than 5 μm across the seeded area. In addition toproviding this flat, scratch-free surface, the chemical reactivity ofthe solution and low hardness (with respect to AlN) of the grit and deckmaterial provides sufficiently low SSD to avoid thermal etching the AlNfoundation.

Following a suitable polishing process, the foundation is chemicallycleaned of polishing residues prior to the described seed-mountingstages involving the foil and seed.

4.1. AlN-to-AlN Bonding Using Al Foil Nitridation

The AlN seed is now bonded to the AlN foundation using Al foilnitridation. The Al foil is placed between the seed and the foundation,and is heated up to temperatures sufficient to nitride the whole Al foiland thus produce a thin AlN bonding film between the AlN seed and AlNfoundation. In other words, the Al foil is interposed between the seedand the foundation, and melted to uniformly wet the foundation with alayer of Al. The Al foil nitridation has the advantages of cleanlinessand producing a microscopically conformal coverage of the seed backside,resulting in low planar-defect densities. The density and chemicalstability of any backing material used to protect the seeds areimportant. If the backing material is not chemically stable (e.g.,against Al vapor), then the resulting reaction between the Al vapor andthe backing material may result in decomposition and thus voiding. Ifthe backing material is not dense enough, the Al vapor can sublimethrough it, leaving behind extended voids and/or planar defects. If thebacking material has a high vapor pressure at AlN crystal growthconditions, then it will migrate allowing void formation and willpossibly become a boule contaminant. A schematic diagram of thisstructure is shown in FIG. 8.

AlN seeds are known to form oxides and hydroxides during exposure toair, moisture and during chemical cleaning (hydrous and anhydrouschemicals contain enough water to react given AlN properties). As such,the prepared and cleaned seed surfaces may have some reproducible layerof oxides or hydroxides present during seed mounting. One advantage ofusing a liquid flux (the aluminum metal is melted and remains a liquidbefore forming a nitride and becoming a solid during the describedprocess) is that the liquid will dissolve the seed surface oxide priorto reaction and convert the oxide into a more stable form and/ordistribution. A layer of oxide and/or hydride on the seed surface mayhave a high vapor pressure under growth conditions and may lead to voidformation. The more chemically reactive side of the AlN c-axis wafer(the N-face) will have hydroxide formation that may be >10 nm inthickness.

The starting materials for an exemplary process are a polished AlNfoundation seed holder, polished AlN seed crystal, and Al foil (10 milthick from Alfa Aesar). First the materials are cleaned to producereproducible and clean surfaces. The AlN foundation seed holder,prepared as described above, is treated as follows:

-   -   1. HCl:H₂O [1:1] boil to remove polishing residues (20 min)    -   2. distilled water rinse    -   3. Room temperature HF (49% solution) dip (15 min)    -   4. Anhydrous Methanol rinse 3 times    -   5. Store under anhydrous methanol while assembling seed mount.    -   6. Dry carefully to avoid solvent stains upon removal from the        anhydrous methanol.

The AlN seed crystal (after preparation as described above) is treatedas follows:

-   -   1. HCl boil to remove remaining epoxy residues from boule        processing (20 min)    -   2. Room temperature HF (49% solution) soak to remove SiO₂ and        polishing residues (15 min) and surface oxide/hydroxide layers.    -   3 Anhydrous methanol rinse 3 times    -   4. Store under anhydrous methanol while assembling seed mount    -   5. Dry carefully to avoid solvent stains upon removal from the        anhydrous methanol.

The Al foil is treated as follows (Al-foil: 10 μm thick, 99.9% purityfoil provided by Alfa Aesar is preferred embodiment):

-   -   1. Cut to square sufficient to cover the seed area    -   2. Drip (1 min) in HF:HNO₃ solution (RT) for 1 min—removes oil        and oxides    -   3. Anhydrous methanol rinse 3 times    -   4. Store under methanol while assembling seed mount    -   5. Dry carefully to avoid solvent stains upon removal from the        anhydrous methanol.        With cleaned components:    -   1. Remove foundation from anhydrous methanol    -   2. Remove Al-foil from anhydrous methanol    -   3. Place foil dull side down and smooth side up onto the        foundation    -   4. Smooth any air bubbles from behind the foil so that the        thin/soft foil is void free on the foundation.    -   5. Remove the seed from the anhydrous methanol    -   6. Place seed (polarity determined) onto the foil    -   7. Trim excess foil from around the seed with a clean razor        blade.

The seed, foil, and foundation are stacked into the furnace (invertedfrom the orientation shown in FIG. 8 to obtain the orientation shown inFIG. 9, so that gravity holds the seed and foil down on the foundation).Clean W weights 900 are then stacked on the seed to ensure that, duringthe melt phase, the seed is pressed toward the mount surface to reducegaps. In an exemplary embodiment, about 0.6 kg of W mass per 2″ wafer.The W weights are cleaned prior to use by heating in a furnace in areducing atmosphere (typically forming gas is used with 3% hydrogen) toa temperature higher than the seed-mounting temperature for severalhours and polished flat by mechanical polishing processes similar to thefoundation and seed process/equipment.

Once the stack of weights, seed, foil, and foundation are positioned inthe furnace, the station is evacuated to base pressures<10⁻⁴ mbar,preferably <10⁻⁶ mbar and refilled with clean gas (filtered UHP gradeforming gas (3% H₂ and 97% N₂). —lower than 1 ppm impurity of moisture,oxygen, hydrocarbons). Preferably, a station is used that is capable ofhigh purity gas flow through the reaction zone where the seed ismounted. The flow gas tends to act as a curtain of clean gas, keepingchamber contamination away from the seed mount area. Contamination ofthe seed mount process may lead to the formation of oxides, carbides,materials other than pure AlN, and pure seed backing material mayintroduce unstable species that may migrate during crystal growth,leaving space that may allow void formation. Seed mount or bondingcontamination (oxide formation) may lead to lower thermal conductivityregions behind/around the seed. Preserving consistent and high qualitythermal contact around the seed and to the seed backing is important formaintaining good seeded growth. Oxides and other impurities tend to havea higher vapor species during crystal growth leading tomigration/sublimation of the contaminant causing void spaces.

As mentioned above, gas flow is one way to improve the purity of theseed mount. A second way is to introduce a getter, with current bestpractice using both gas flow and getter materials. The preferredgettering materials are yttrium metal and hafnium metal. These act togetter the local atmosphere of contamination around the seed duringmounting. The yttrium metal melts at 1522° C. (during ramp up of theAl-foil seed mounting process) and spreads to getter a wide surfacearea. Using a thin foil of the material tends to be most effective(e.g., Alfa Aesar, 0.1 mm thick, 99.999% purity Y-foil). Furthermore,yttrium oxide is stable under typical AlN growth conditions, meaningthat it will provide only a low vapor pressure of oxide contaminationback into the crystal growth environment if this getter from the seedmount remains during the growth. Hafnium-metal getter will not melt(melting point>2200° C.) under the described seed mount conditions buttends to surface react with both the oxide and the nitrogen. Therefore,the powder form of hafnium is preferred for this application (e.g., AlfaAesar, −325 mesh, 99.9% metal basis purity). Each of these getters canbe cleaned prior to use or purchased in sufficient purity to be used forthe described application (99.9% or purer is current practice).

In each case, getter materials are placed around the periphery of theseed mounting area at the edges of the seed holder to avoid impuritiesfrom entering the seed bonding reaction zone.

In the case of hafnium powder, the hafnium will readily nitride underthe described process. The HfN layer created in at the powder level orat higher temperatures (when the Hf melts and spreads at 2205° C.)forming a HfN layer. It has been observed that the HfN layer acts toprevent W-components from sticking together, even following long heatingcycles with Al-vapor present. This property allows surfaces to beprepared that will not stick together, despite being well polished andvery clean in the hot/reducing atmosphere.

After these steps, the seed mounting setup is ready for the heatingcycle. In a preferred embodiment, the seed-mount stack is rapidly heated(<5 minutes) to approximately 1600° C. and ramped in 30 minutes to 1650°C. The purpose of this is to quickly melt the Al foil and to allow theAl liquid to readily flow with low surface tension, allowing the Al meltto readily wet the AlN seed crystal and the AlN foundation uniformly,i.e., melting the Al foil to uniformly wet the foundation with a layerof Al. A high density AlN between the original seed crystal and the AlNfoundation is formed. Allowing the heat-up cycle to remain at lowtemperatures (below about 1100° C.) for too long may permit the liquidAl to bead up and form a porous AlN ceramic when the Al starts tonitride, thereby creating void spaces behind the seed. Once at 1650° C.,the temperature is held for >1 hour to allow the Al-melt to fullynitride, forming a high-density AlN ceramic that is bonded to the seedand to the AlN foundation. Following the >1 hour soak at 1650° C., thestation is ramped to room temperature in an additional 2 hours.

Following this heat cycle/nitride mounting, remaining getter materialsand seed mounting weight 900 are removed from the assembled seed-seedholder. The seed and seed holder assembly is now ready to be inverted asshown in FIG. 8 and assembled for the crystal growth cycle. The crystalgrowth crucible is assembled as shown in FIG. 10. In particular, AlNseed 100 and seed holder assembly (including adhesive 140, foundation810, and backing plate 820) are assembled as shown in a crystal growthcrucible 1100. The AlN seeded bulk crystal growth may be carried out ina tungsten crucible 1100 using a high-purity AlN source 1120. The AlNseed 100 is mounted onto the seed holder assembly as described above.

Single-crystal aluminum nitride is formed by depositing aluminum andnitrogen onto the AlN seed 100 under conditions suitable for growingsingle-crystal AlN originating at the seed. For example, growth may beinitiated by heating the crucible with the seed mount and sourcematerial to a maximum temperature of approximately 2300° C. and with agradient of less than 50° C./cm as measured radially and a verticalgradient greater than 1° C./cm but less than 50° C./cm. During theinitial ramp-up to the growth temperature, it may be desirable toposition the seed crystal and the source material such that they are atapproximately the same temperature (the seed equilibrium position) sothat any impurities on the surface of the seed crystal are evaporatedaway prior to growth. Once the growth temperature is achieved, it may bedesirable to either move the crucible assembly so that the seed istemporarily hotter than the source material, or to temporarily reducethe nitrogen partial pressure prior to initiating growth on the seedcrystal in order to evaporate part of the surface of the seed crystal.The partial pressure of nitrogen in the furnace may be reduced either byreducing the total pressure of gas in the furnace or by adding an inertgas, such as Ar, to the furnace while keeping the total pressure in thefurnace constant.

The bulk single crystal of AlN formed by this method may have a diametergreater than 20 mm, a thickness greater than 0.1 mm, and an areal defectdensity≦100 cm⁻². The method may enable the formation of bulk singlecrystal AlN in the form of a boule having a diameter greater than 20 mm,a thickness greater than 5 mm, and an areal density of threadingdislocations≦10⁶ cm⁻²—or even ≦10⁴ cm⁻²—in each cross section of thebulk single crystal disposed in a plane perpendicular to a growthdirection of the crystal. A boule may include a bulk single crystal ofAlN having a sufficient thickness to enable the formation of at leastfive wafers therefrom, each wafer having a thickness of at least 0.1 mm,a diameter of at least 20 mm, and a threading dislocation density≦10⁶cm⁻², preferably ≦10⁴ cm⁻².

A boule formed by methods described herein may be a substantiallycylindrical bulk single crystal of AlN having a diameter of at least 20mm and having a sufficient thickness to enable the formation of at leastfive wafers therefrom, each wafer having a thickness of at least 0.1 mm,a diameter of at least 20 mm, and a triple-crystal X-ray rocking curveof less than 50 arcsec FWHM for a (0002) reflection, with each waferhaving substantially the same diameter as each of the other wafers.

4.2 Multiple Seed Mounting

It may be desirable to mount several seeds on the AlN ceramicsimultaneously. For instance, it may be difficult to obtain seedcrystals large enough, with sufficiently high quality, to cover theentire area of the AlN ceramic. In this case, it may be desirable to usemultiple seeds that may be mounted on the AlN ceramic simultaneously.This may be accomplished by preparing seed crystals as described above,all with the same orientation. The seed crystals may then be mounted onthe AlN ceramic on the metal backing plate as described above (or otherseed holder assemblies as described below) with careful attention toaligning their azimuthal axis. In the case of smaller seeds, it ispossible to expand a seed within a growth run using thermal gradients.The laterally expanded seed crystal generally avoids the seed mountingsource of planar voids but may still require a low porosity seed backingbarrier to avoid through voiding formation of planar defects within thegrown boule. In addition, it may be possible to arrange a patch work ormosaic of small seeds accurately enough so that the resulting largediameter boule is grown with suitable orientation between the smallerseeded regions to produce a congruent 2″ wafer. For c-axis AlN seededgrowth, the alignment of the seeds is preferably accomplished bypreparing the seed crystals with m-plane cleaved edges. The AlN cleaveson the m-plane to produce very straight edges perpendicular to thec-axis. Thus, the seeds may be well oriented with respect to each otherby aligning the flat m-plane cleaves against the neighboring seedsections. From a small seed mosaic approach, a fraction of the 2″ waferusable area may easily be produced, but it may also be possible to seedthe entire 2″ area by this method. A particularly important example ofusing more than one seed crystal is when a 2″ seed crystal is crackedand this seed crystal is mounted with the two halves aligned preciselyfor boule growth. By using this m-plane cleave face alignment approachto c-axis seeded growth it is possible to achieve <0.5 deg m-plane andc-axis crystallographic alignment. Because of the difficulty inobtaining seed crystals that are all exactly aligned and the difficultyin avoiding some error in the aligning of the azimuthal axes, thisapproach typically produces a higher defect density than a single seedcrystal. However, this approach may be used to obtain larger AlN crystalboules with smaller seed sizes.

4.3 Additional Approaches that May be Used to Supplement the PreferredImplementations

4.3.1 Protection of the AlN Seed Using Relatively Impervious Films

The back of the AlN seed may be protected by depositing ahigh-temperature, relatively impervious material like W. This barrierlayer can be deposited by sputtering, CVD, ion deposition or plating(for conductive substrates). Plating may be used to initiate or thickenthe deposited layer of seed back sealant once initial deposition hasbeen performed. For instance, the back of the AlN seed can be protectedusing W film sputtered onto the back of the AlN seed and then mounted tothe seed holder using any of the techniques described above. The back ofthe AlN seed may also be protected by attaching it (with an adhesivesuch AlN which is formed by nitriding a thin foil of Al as was describedabove) to W foil. The W foil may be single crystal to reduce Aldiffusion. The density of the planar defects is then reducedsignificantly. Other materials expected to posses suitable properties tobe used as relatively impervious barriers include: Hf, HfN, HfC,W—Re(<25%), W—Mo(<10%), pyrolitic-BN (also called CVD-BN), Ta, TaC, TaN,Ta₂N, Carbon (vitreous, glassy, CVD, POCO) and carbon coated withTa/TaC, Hf/HfC and BN. The key attributes of a suitable material to bedeposited on the back surface include:

-   -   a. Temperature stability (>2100° C.)    -   b. Chemically stable in growth environment (Al-vapor, N₂, H₂)—        vapor pressures<1 mbar at temperatures>2100° C. in N₂,        N₂—H₂(<10%), Ar, around 1 atm pressure.    -   c. Low diffusivity of Al through the backing material by being        physically impervious to gas flow (generally this means that the        material is dense without voids) and having a small diffusion        constant for Al. Since diffusion along grain boundaries is        generally much higher than diffusion through grain boundaries,        it may be desirable to have the grains swell so as to become        more dense as Al diffuses into the material (“self-sealing”        grain swelling as described in the '027 application.)

The material may, for example, be exposed to Al vapor prior to use as aseed holder plate to limit Al diffusivity through grain swelling in theplate. At typical growth temperatures, the vapor pressure in the growthatmosphere is about 0.1 bar Al-vapor and the equilibrium (atom-wt-%) Allevel in W has been measured to be ˜5%, so the preferred backing willhave no voids, will not evaporate or migrate during the run, and willhave its surface pre-saturated with the equilibrium Al-content for thatmaterial at the anticipated growth temperature.

4.3.2. Growth of Bulk AlN Single-Crystals Along Off-Axis Directions

The AlN bulk crystal may be grown parallel to directions at least 15±5°off-axis. The off-axis) growth include crystal growth with interfaceparallel to non-polar {1 100} and semi-polar planes {10 11}, {1 102},and (10 13). In the case of non-polar growth, the growth rate of thecrystallographic planes differs from the growth rate of the same planeswhen crystal is grown on-axis or slightly off-axis. Therefore, eventhough the back surface of the seed may not be perfectly protected,planar-defect formation may be resolved into generation of otherdefects, e.g., stacking faults, twinning, etc., to reduce its impact.

4.3.3 Protection of the Back of the Backing Plate (Outer Sealing)

In addition to mounting the AlN seed onto the seed holder as describedabove, the outside of the seed holder (i.e., backing plate 820 in FIG.8), that forms the crucible lid—i.e., the side outside the crucible—maybe protected to inhibit the transport of Al through the crucible lid.For purposes herein, high-temperature carbon-based adhesives, paints, orcoatings may be applied. Typically these materials are applied bybrushing or spraying and then thermally cycled to improve their densityand structure, but they may be sputtered or electro-plated as well. Forinstance, if a thin (<0.005 inch) W foil is used as a crucible lid andwith the AlN seed mounted on one side, then the other side of the W foilmay be protected in this fashion. The advantage of protecting the outerside of the foil is that a much wider range of high-temperaturematerials (coating, paints, etc.) may be used as the protective layer,since there is a lower risk of interaction between Al vapor and theprotective material. This approach also allows thinner metal lids to beused, which is advantageous in reducing stress on the crystal due tothermal-expansion mismatch between the lid material and AlN.

4.3.4 Seed Bond Curing in Multiple Gas Species Flow

As mentioned above, within at typical Al-foil seed mounting process, theliquid Al-foil cleans the seed surface of oxides and reacts to formAl₂O₃. To move to fewer voids and better quality growth, it may benecessary to more fully remove this seed oxide layer. Extending the timethat the Al-foil melt is allowed to react with this oxide layer is onemethod for doing this. The longer Al-melt phase may be achieved byreducing the amount of nitrogen available to react with the moltenAl-metal forming solid nitride. This can be performed under an argonatmosphere during heat up to suitable reaction temperature (1000 to1800° C. depending on desired removal rate/species) and holding forsufficient time to remove oxide and hydroxide layers from the seed.Subsequently, nitrogen may be added to the flow past the seed mountzone. The nitrogen may then react with the free Al-melt and form anitride seed adhesive.

During this molten Al phase, it is possible that the seed holder (whenmade of W alone) will be a diffusion membrane for the oxide species.This mechanism would allow the getter of the oxide from the seed to beachieved by the Al-metal, the metal to be cleaned by the W-layer andthen pure, high density AlN to be nitrided from the Al-melt forming ahigh quality seed adhesive.

4.3.6 Seed Bonding Directly to a Seed Plate without the AlN Layer

Rather than using a combination of an AlN ceramic layer and a backingplate, it may also be possible to bond the seed directly to anappropriate seed plate without the intermediary AlN ceramic layer. Thismay provide the advantage of eliminating the potential for defects inthe AlN ceramic layer to migrate into the growing AlN boule. However,the backing plate is carefully chosen so as to not introduce too muchstress onto the seed crystal and AlN boule due to thermal expansionmismatch between the seed plate and AlN. This can be accomplished eitherby using very thin plates that will easily deform in response to stressfrom the AlN crystal (yet still be relatively impervious to Al transportthrough the plate) or by using plates that relatively closely match thethermal expansion of AlN from room temperature up to the growthtemperature of ˜2200° C. Alternatively, the AlN seed crystal may bemounted on the backing plate, which may then be mounted on a texturedAlN ceramic. This last approach is attractive because the seed backingplate used may provide a relatively impervious barrier to Al diffusionand prevent defects from the AlN ceramic from diffusing into the growingcrystal. However, the AlN ceramic may provide the mechanical strength tohold the growing crystal boule.

Possible choices include:

-   -   i. W-foil    -   ii. W—Re foil    -   iii. W—Mo foil    -   iv. W-foil treated with Pt, V, Y, carbon (crucible patent        reference)    -   v. Single crystal-W backing    -   vi. HfC—liquid phase sintered    -   vii. TaC coated Ta    -   viii. TaC coated pBN    -   ix. TaC coated W-foil    -   x. HfN coated W-foil    -   xi. HfC (hafnium carbide)    -   xii. HfC coated W    -   xiii. BN coated graphite

Even though the W has a thermal expansion coefficient different fromthat of AlN, thin W-foil and thin single crystal-W may mechanicallydeform much more readily than an AlN boule of suitable thickness so asto reduce stresses on the crystal due to the thermal expansion mismatch.Alloys of W/Re and W/Mo may be selected such that the total thermalexpansion of the seed holder and AlN will be zero from growthtemperature down to room temperature. Combinations of these materials(all) and treatments with elements such as Pt, V, Y, carbon may be usedto change the grain growth behavior of the backing material to reducethe time dependent grain growth of the material upon exposure to Al andhigh temperature gradients.

A similar polishing preparation process to what was described above forthe AlN ceramic foundation is also suitable for direct foil mounting(without AlN foundation). To improve the surface finish further in thecases of metal backing it is generally desirable to follow the 1 μmAl₂O₃ deck step with a 1200-grit pad step that produces a mirror finishon the softer metal materials while maintaining flatness and lowscratching.

The furnace operation for this seed mounting process is schematicallydescribed below. The adhesive layer is place on the prepared seed holderand the seed onto the adhesive layer. For use of the Al-foil based seedmounting adhesive, the seed holder from FIG. 6 is assembled whileinverted within a station capable of reaching at least 1650° C. Formaterials other than Al-foil, a separate heating cycle is describedlater, however, the same considerations apply to maintaining highquality seeded growth results.

A suitable mass is placed on top of the seed/adhesive/seed holderassembly. In an embodiment, one may use a polished (flat) tungsten rightcylinder that has been carefully out-gassed of contamination by repeatedheating cycles under forming gas flow. The block presses on the polished(flat) seed face with a pressure greater than 150 grams per centimetersquared area. In this case, this may be sufficient to hold a flat,stress relived seed closely against the seed holder. More pressure perarea will help to improve imperfect seed/seed-holder flatness bydeformation of the materials up to the point where the mass loading maycause seed/seed-holder fracture by exceeding the critical resolved sheerstress (CRSS) at room or higher temperatures.

Prior to seed assembly, the seed and seed holder are typically checkedfor suitable flatness using optical flatness measurement techniques suchas an optical flat and a monochromatic light source (435 nm sodiumlamp). Gaps between the mating surfaces are preferably less than 5 μm,preferably less, with part shapes being regular (avoid cupped or boxedpieces with deformation better than 5 μm preferred).

4.3.7 Other Possible Seed Mounting Adhesives

Instead of an AlN ceramic-based adhesive, it is possible to use anyother high-temperature adhesive, e.g., carbon-based adhesives or evenwater-based carbon paints such as Aquadag E, molybdenum-dag, (such asfrom Aremco Products, Inc.) molybdenum-powder or foil, molybdenumsputter or plated coatings, similar to each of the molybdenum formsincluding base elements aluminum, rhenium, vanadium, yttrium. Otherglues, such as boron nitride-, zirconia-, yttrium oxide-, and aluminumoxide-based glues that have a variety of high temperaturestabilities/suitability at AlN growth conditions may also be used.

The carbon-based approaches have been successful for seeding SiC crystalgrowth. However, they have not proven successful for AlN crystal growthbecause Al vapor attacks the graphite forming aluminum carbide (Al₄C₃).

4.3.8 Using a Liquid or Break-Away Seed Mounting

As discussed above, one of the difficulties of growing bulk AlN fromseed crystals mounted on seed holders that are nearly impervious to Altransport is the strain caused by the thermal expansion mismatch betweenthe seed crystal and the seed holder plate. Stress from thermalexpansion mismatch can be avoided by using a liquid or nearly liquidfilm to hold the seed to the seed holder plate. Metal gallium (Ga) maybe substituted for one of the solid glues described above and will meltat 30° C. At high temperatures (>1,000° C.), the nitrides of Ga are notstable so the Ga will remain liquid between the AlN seed and the seedholder plate and thus will not be able to transmit any shear stress (dueto thermal expansion mismatch) to the growing AlN boule. However, theliquid Ga typically forms a nitride as the crystal is cooled to roomtemperature. This may be avoided by using a backing plate from which theGaN will break away as it cools or by replacing the nitrogen gas in thegrowth chamber with an inert gas (such as Ar) so that the Ga will not beexposed to enough nitrogen to form a solid nitride bond both the seedcrystal and the seed holder plate. Of course, this approach may notprovide any mechanical strength to hold the seed crystal, so it ispreferably used by mounting the seed crystal at the bottom of the growthcrucible.

The relatively high vapor pressure of the Ga may cause contamination ofthe growing AlN crystal boule. This may be overcome by using a eutecticof gold and germanium. The Au_(x)Ge_(1-x) has a eutectic at x=0.72 whichmelts at 361° C. Again, this material does not have any stable nitridesat the AlN growth temperature and, thus, will remain liquid. Inaddition, its vapor pressure will be approximately 30 times lower thanthat of Ga at the same temperature.

4.3.9 Seed Mounting without a Holder Plate

A large, low defect seed crystal may also be mounted by coating its backsurface with a nearly impervious coating and using the seed crystalitself to seal the crystal growth crucible. By making this coating thin,mechanical stresses from the thermal expansion mismatch between thecoating and the AlN seed crystal will be minimized. In the preferredembodiment of this approach, the seed crystal is first coated in DAG andthen baked at 150° C. to provide a carbon coating around the entire seed(alternative carbon coating schemes may also be used). The carbon coatedAlN seed crystal then has a thin layer of pyrolytic BN deposited on it(this layer is preferably approximately 100 μm thick). After thispreparation, the front surface of the AlN seed crystal is polished asdescribed above in the section on seed crystal preparation, so that thefront surface has substantially all of the BN and graphite removed, andis smooth and relatively defect-free as described in that section. Thisintegrated seed crystal and seed holder assembly will then be mounteddirectly as the lid for the AlN crystal growth crucible.

Those skilled in the art will readily appreciate that all parameterslisted herein are meant to be exemplary and actual parameters dependupon the specific application for which the methods and materials of thepresent invention are used. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto, theinvention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A method for growing single-crystal aluminum nitride (AlN), the method comprising: providing a holder sized and shaped to receive an AlN seed, the holder comprising a substantially impervious backing plate; disposing an AlN seed and a foil on the backing plate with the foil between the AlN seed and the backing plate; and depositing aluminum and nitrogen onto the AlN seed under conditions suitable for growing single-crystal AlN originating at the AlN seed.
 2. The method of claim 1, wherein the backing plate is substantially impervious to aluminum transport.
 3. The method of claim 2, wherein the backing plate is substantially impervious to nitrogen.
 4. The method of claim 1, wherein the backing plate is substantially impervious to nitrogen.
 5. The method of claim 1, wherein the foil is substantially impervious to aluminum transport.
 6. The method of claim 1, wherein a surface of the AlN seed facing the holder has a total thickness variance of less than 5 μm.
 7. The method of claim 1, wherein a surface of the AN seed facing the holder has a root mean square roughness less than 1 nm in a 10 μm×10 μm square area.
 8. The method of claim 1, wherein providing the holder comprises exposing the backing plate to an Al vapor.
 9. The method of claim 1, wherein the backing plate comprises multiple layers of grains, at least some of which are swollen to inhibit diffusion therebetween.
 10. The method of claim 1, wherein the backing plate consists essentially of a single-crystal material.
 11. The method of claim 1, wherein the backing plate is mechanically flexible.
 12. The method of claim 1, wherein the backing plate comprises tungsten.
 13. The method of claim 1, wherein the backing plate consists essentially of tungsten.
 14. The method of claim 1, wherein the foil comprises tungsten.
 15. The method of claim 1, wherein the foil consists essentially of tungsten.
 16. The method of claim 1, wherein the foil comprises aluminum.
 17. The method of claim 1, wherein the foil consists essentially of aluminum.
 18. The method of claim 1, further comprising disposing a barrier layer over at least a portion of a surface of the AlN seed.
 19. The method of claim 18, wherein the barrier layer comprises at least one of tungsten, Hf, HfN, HfC, W—Re, W—Mo, BN, Ta, TaC, TaN, Ta₂N, or carbon.
 20. The method of claim 18, wherein the barrier layer consists essentially of tungsten.
 21. The method of claim 1, wherein the AlN seed is a wafer having a diameter of at least 22 mm.
 22. The method of claim 1, wherein the grown single-crystal AlN comprises a boule having a diameter approximately the same as a diameter of the AlN seed.
 23. The method of claim 1, wherein the AlN seed is substantially free of planar defects.
 24. The method of claim 1, wherein the AlN seed is substantially free of subsurface damage.
 25. The method of claim 1, further comprising, prior to disposing the AlN seed on the backing plate, polishing a surface of the AlN seed to at least one of reduce a surface roughness of the AlN seed or remove subsurface damage from the AlN seed.
 26. The method of claim 1, further comprising applying an adhesive to at least a portion of the backing plate.
 27. The method of claim 1, wherein the grown single-crystal AlN has a diameter greater than 20 mm, a thickness greater than 0.1 mm, and an areal planar defect density≦100 cm⁻².
 28. The method of claim 27, wherein the areal planar defect density is ≦1 cm⁻².
 29. The method of claim 1, wherein (i) the deposition on the AlN seed occurs within a crystal growth crucible, and (ii) the backing plate is a portion of the crystal growth crucible.
 30. The method of claim 1, wherein the backing plate comprises powder metallurgy tungsten.
 31. The method of claim 1, wherein the foil comprises polycrystalline tungsten.
 32. The method of claim 1, wherein (i) the aluminum and nitrogen are deposited onto the AlN seed at a growth temperature, and (ii) in a temperature range of approximately room temperature to the growth temperature, thermal expansion of at least one of the holder or the backing plate is approximately equal to thermal expansion of the AlN seed.
 33. The method of claim 1, wherein at least a portion of the foil melts before and/or during the aluminum and nitrogen deposition. 